Conventionally, a motor driving device including a plurality of switching elements is known. For example, JP-A-2011-142744 (corresponding to US 2011/163708 A1) discloses a motor driving device that includes a plurality of switching elements and a power source relay. The switching elements form a motor driving portion. The power source relay cuts off power supply to the motor driving portion when a failure occurs in the motor driving device. The switching elements and the power source relay are driven through pre-driver circuits.
In a case where a semiconductor device having a parasitic diode, such as a metal-oxide semiconductor field-effect transistor (MOSFET), is used for the power source relay, in order to protect a circuit in case the power source is incorrectly connected in reverse polarity, a reverse connection protection relay may be disposed such that a parasitic diode of the reverse connection protection relay is connected in an opposite direction.
In a case where a motor is applied to an electric power steering apparatus and a steering wheel is directly connected to the motor, an induced voltage is generated in the motor when the steering wheel is operated by a driver. When a failure of the motor occurs and the power source relay is turned off, the induced voltage generated due to a steering operation by the driver is applied to a closed circuit formed between an inverter portion and a winding group. When the induced voltage is applied to the closed circuit formed between the inverter portion and the winding group in a state where the power source relay is in off-state, electric current may flow to the closed circuit to generate a brake torque and the driver may feel uncomfortable. Thus, a motor relay that is capable of cutting off an electrical connection between the winding group and the inverter portion may be disposed so that the closed circuit is not formed between winding group and the inverter portion when a failure of the motor occurs.
The switching elements and the relays may be driven through the pre-driver circuits. However, JP-A-2011-142744 does not disclose a detail of the pre-driver circuits. Although pre-driver circuits may be provided respectively to the switching elements and the relays, a size of the motor driving device may increase.